in training for my first half, I didn't take in any fuel at all. I just went out and did it. I made it through, but that also set me up for years of not taking in enough fuel during races. After that race, I would take an energy chew every 3 miles or so or whenever I realized I should have taken one, but didn't. This did not cause me much trouble in the half marathon distance, but it would have been disastrous during my first marathon.
For me, marathon training exposed just how poorly I had been doing at taking in fuel during long runs. I was hitting walls and bonking in the middle of distances that normally caused no trouble at all for me. It wasn't until I began to review everything with
@DopeyBadger that I realized I was taking in 25% or less of what I should have been taking in.
I use the 90+ minute rule. If I will be running for more than 90 minutes, I take in fuel roughly every 45 minutes, starting 45 minutes after the run begins. Because we are all different, our bodies respond differently to different types of fuel. When I began to increase my race fuel to where it should be, I quickly discovered that it usually wasn't sitting as well on my stomach as I would like. So while I really like the taste of sport beans, I only use them during races when they're given out. In small quantities they work great for me, but if they are my only fuel source during a long run, they don't settle wonderfully on my stomach. I've never had serious issues, but it's a comfort thing.
I settled on huma gels. They work really well for me and I experimented with different flavors in marathon training so I knew what I really liked and reserved those flavors for certain moments.
I continued to stick with the every 45 minute rule, but made sure I was taking in enough fuel.
While I sometimes think I feel some energy kick in a few minutes after taking the fuel, I'm not entirely sure that's an actual direct result of the fuel. The placebo effect could be very real. The biggest measurable that I can certainly see is sufficient energy to keep going. When fueling properly I don't feel like I'm dragging as much during long runs/races.
I've used clif bloks, sport beans, and huma gels. I'm fine with the taste of all of those, but find that huma gels work best as consistent fuel for me. Sport beans offer a nice break and sort of kind of feel like something different, which I tend to want in longer runs.
Definitely the case for me. You need to have a very good idea of how your body responds to different types of fuels so that new fuel does not cause major trouble on race day.